Eternal Sunshine's title comes from a line in Alexander Pope's poem, "Eloise to Abelard," which Mary Svevo recites to Howard during the memory-erasing procedure. The lines that Mary recites are:
How happy is the blameless vestal's lot!
The world forgetting, by the world forgot.
Eternal sunshine of the spotless mind!
Each pray'r accepted, and each wish resign'd […].
Before we dig into the lines themselves, let's talk about the poem. Pope based it on a story of 12th-century lovers Abelard (a famous French philosopher) and Heloïse, one of his pupils. The short version is that they fell in love and got married in secret, and when Heloïse's family finds out, they're not too happy, so they castrate Abelard, because what else are they supposed to do?
The poem is what's called an Ovidian heroic epistle, so let's break that down before we continue. "Epistle" refers to the fact that the poem is written as a letter (hence the title "Eloise to Abelard"). Ovid, a Roman poet, wrote many poems called "heroides," just like Pope's; they were in the form of letters women addressed lovers who had somehow wronged them. And, of course, you can't forget about the rhyming couplets that also define the genre.
When Wishes Become Reality
But enough history—Pope's poem is a long 'un, so we'll give you a super short summary. Essentially, Eloise is caught in a bind and doesn't know what to do. She still has very strong feelings for Abelard, but he has asked her to take on the life of a nun, which she does unwillingly. Since Abelard is now a eunuch, Eloise worries that he cannot return her love, and she's stuck between a life with him that she can't have and a religious life that she doesn't want.
And this is where these lines come in. Eloise is envious of the "blameless vestal," or virgin. Specifically, "vestal" refers to the Roman goddess Vesta (Hestia in Greek) and the priestesses who served her, who would take a vow of chastity. These women are both forgotten by the world and able to forget the world. Their existence is defined by their loyalty to Vesta, and thus they are blameless, sinless, and chaste.
So, the "spotless mind" in this context isn't necessarily a blank mind, as it seems to be in the film; it refers instead to an innocent mind, a mind that does not have to account for its own sin. Virgins with their spotless minds have everything they want: they are eternally happy—or at least that's how the destitute Eloise sees it.
Of course, it's more complicated than that, because the virgins achieve what seems to be happiness only by abstaining from some pretty big parts of life. Is it really happiness, or is it escapism? Put differently, is it better not to know pain or to know it?
In the film, of course, the emphasis is not on sin, exactly, but more on memory and lack of worldly connection. Joel and Clem use Lacuna to withdraw from the life they had and the person they shared it with. But let's look at Mary for a second. Her affair with Howard has been erased from her mind. Her mind is spotless because it is sinless, and it is sinless because it is blank. She does not have to deal with the memory of the affair since the Lacuna procedure.
It's possible to read Mary's recitation of Pope as ironic, because having the procedure only causes her more pain when she learns the truth. But really, learning the truth is the real problem. Only when the effects of the procedure are somewhat undone by listening to the tape does Mary experience the pain of her spotless mind becoming tainted again and her eternal sunshine disappearing behind the horizon of truth.
So that's the big question: is it better to know, or not to know? Is ignorance bliss? Or is the pain of knowledge worth it—as long as you learn how to deal with it?